1. Field of the Invention
The present invention relates to high-temperature piping products for power plants, particularly to high-temperature piping products used in coal-fired power plants and gas turbine power plants, and methods for producing such piping products.
2. Description of Related Art
In order to improve the power generating efficiency of power plants such as coal-fired power plants and gas turbine power plants, it is effective to increase the main steam temperature of a boiler and the combustion temperature of a gas turbine. In this connection, piping products with higher tolerable temperature are needed to further increase the main steam temperature or the combustion temperature.
For example, coal-fired power plants using a steam turbine include large scale high-temperature piping products for the boiler. Such large scale high-temperature piping products are assembled usually by welding short pipe members. In the power plants which produce a main steam of a temperature of around 700° C., an Ni-based alloy precipitation-strengthened with γ′ phase (Ni3Al phase) is used as a material of the high-temperature piping products.
In an Ni-based alloy, it is indispensable that the γ′ phase is evenly dispersed and precipitated in a matrix phase by heat treatment to attain high mechanical strength at a high-temperature. On the other hand, precipitation of the γ′ phase is detrimental to weldability, and makes the welding difficult. It is accordingly desirable to perform the heat treatment after the welding. A problem, however, is that, because the welded and assembled high-temperature piping product is a large structure, it is very difficult after assembling to conduct a heat treatment precipitating the γ′ phase (an aging heat treatment, so called) to the whole piping product.
As a countermeasure, WO2009/028671 reports a technique whereby boiler components are welded and assembled without precipitating the γ′ phase, and the γ′ phase is evenly dispersed and precipitated in the matrix phase of the boiler component by heat generated through a power plant operation. More specifically, the publication describes a low-thermal-expansion Ni-based super-heat-resistant alloy preferred for use as a boiler component material. The alloy contains, in mass %, C of 0.2% or less, Si of 0.5% or less, Mn of 0.5% or less, Cr of 10 to 24%, one of or both of Mo and W in amounts specified by the equation of “Mo+0.5 W=5 to 17%”, Al of 0.5 to 2.0%, Ti of 1.0 to 3.0%, Fe of 10% or less, and one of or both of B (0.02% or less; excluding 0%) and Zr (0.2% or less; excluding 0%), with the remainder being Ni and unavoidable impurities. The alloy has a Vickers hardness of 240 or less.
The assumed operating temperature of the Ni-based alloy (or the boiler component) described in WO2009/028671 ranges from 700 to 750° C. (700° C. class), and the precipitation amount of the γ′ phase is about 20%. In response to the recent strong demand for further improvement of the power generating efficiency of power plants, there have been studies directed to increasing the main steam temperature or combustion temperature to above 750° C. and as high as about 800° C. (800° C. class). In order to increase the tolerable temperature of the high-temperature piping product in the power plant to temperatures of the 800° C. class, there is required a γ′-phase precipitation amount of 30% or more in the material Ni-based alloy.
In one known method of attaining a γ′-phase precipitation amount of 30% or more to obtain an effective high-temperature strength for the Ni-based alloy, for example, a heat treatment is performed at 900° C. or more to precipitate about 10% of the γ′ phase, followed by an aging heat treatment at 700 to 800° C. However, as described above, it is difficult to perform such a series of heat treatments for the welded and assembled high-temperature piping product. Furthermore, it is revealed through studies conducted by the present inventors that it is difficult to ensure the required high-temperature strength characteristics by the direct application of the Ni-based alloy described in WO2009/028671. (This will be described later in greater detail.)